Since I have recently moved up to the big dogs (no more pulling battery out to charge after each trip) I have a question.

How long can I expect it to take for the batteries to charge fully on a three battery Triton setup?

I suppose that would depend on how much amperage is put out per bank.  I just recently got an onboard charger too, haven't installed it yet but I'm using it here in the house until the next warm day.  Mine puts out 5 amps per bank, dual bank, and it's owner's manual says that mine would take 10 - 12 hours to charge a discharged battery fully.  I bought a Guest 2611 dual bank charger.  Puts out 5/5 for 10 amps total.  Interestingly, it also says I can parallel wire the banks for 10 amps output.  I'm guessing that would be into one battery??  The wiring diagrams didn't show that pic so I probably won't try it.  

My charger stays on 24/7/365.  Once the batteries are charged it switches to trickle mode, only coming on if the battery charge warrants it.

In a perfect world you could take the amount of amps drawn from the batteries and divide that by the amp output of the charger and get the answer.  One way to find out is to get a hydrometer and check the specific gravity in the battery cells as they are charging.  Once it reaches 12.7 v you will know the amount of time.